Final answer:
Retroviruses can carry oncogenes that may lead to tumor formation by stimulating unregulated cell growth or by interfering with the expression of genes that inhibit cell growth. These oncogenic viruses include DNA or RNA viruses and can result in cancers such as cervical, liver cancer, T-cell leukemia, and lymphomas.
Step-by-step explanation:
Some retroviruses have associated oncogenes, or cancer genes. Oncogenes can stimulate cell growth and lead to tumor formation, a process called oncogenesis or tumorigenesis. Oncogenic viruses can be either DNA or RNA viruses and are known to disrupt normal cell cycle regulation.
This can occur by the virus introducing oncogenes that stimulate unregulated cell growth or by interfering with genes that inhibit cell growth. Examples of cancers associated with viral infections include cervical cancer from human papillomavirus (HPV), liver cancer from hepatitis B virus, T-cell leukemia, and various lymphomas.
Oncogenic viruses may induce cancer through acutely-transforming viruses that carry a viral-oncogene (v-onc), resulting in immediate cell transformation upon expression. In contrast, slowly-transforming viruses can integrate their genome near a proto-oncogene in the host genome, causing overexpression of that proto-oncogene and subsequent uncontrolled cellular proliferation.
The integration of the viral genome, a characteristic of retroviruses, can cause the expression of proto-oncogenes to become dysregulated, leading to tumorigenesis with a longer latency period.
Viruses like the simian virus 40 (SV40), which was a contaminant in polio vaccines from the 1960s, show that viral infections can alter the regulation of cell cycle-related proteins such as p53, causing the host cell to divide uncontrollably and potentially form tumors.